In a base station cellular telephony system comprising transmitter and receiver units, the output of the transmitter and the input of the receiver consist, respectively, of signals in multiple channels included within a defined transmitted band of frequencies and signals in multiple channels included within a defined received band of frequencies separated by a gap in the frequency domain from the transmitted band. Such system transmits and receives signals simultaneously.
Because of the simultaneity of occurrence of signals transmitted from and received by such a system and the consequent danger of interference between these two kinds of signals, it was common practice in the past for such systems to be a simplex system in which the transmitter and receiver each had its own antenna, and the two antennas were spaced apart by a distance far enough to prevent any such interference from occurring to a significant degree.
As a cost saving measure however, the art has recently turned to instead of such simplex systems a base station cellular telephony system in which one of these two antennas is eliminated, and the systems becomes a duplex system in which the one remaining antenna is a common antenna for both the transmitter unit and the receiver unit. In such a duplex system those two units and the one antenna are interlinked through a microwave plumbing assemblage comprising a duplexer and first and second interdigital bandpass filters which are respectively coupled to the transmitter's output and the receiver's input.
The duplexer comprises a "T" junction and three coaxial lines all coupled at one of their ends to such junction, a first and second of such lines being coupled at their ends away from such junction to, respectively, the output of the first filter and the input of the second filter, and the third of such lines being coupled at its end away from that junction to the common antenna. The first and second filters are designed to pass, respectively, the transmission band and the reception band and to reject signals outside of the pass band of the filter. Further, the microwave assemblage is designed to, in effect, steer signals from the transmitter to the antenna but not to the receiver and, simultaneously, to steer signals received by the antenna to the receiver but not to the transmitter. In this way, the system is intended to prevent the transmitted signals from interfering with the signals received by the receiver, and conversely.
When, however, experimental field trials were recently made of base station cellular telephony duplex systems wherein the plumbing assemblages incorporated components made in accordance with prevailing commercial practices, it was found that the sensitivity of the receiver was degraded by the presence at its input of an inordinately high level of electromagnetic interference. Such interference was in the form of intermodulation products of frequencies lying within the reception band and generated by signals in different channels in the transmitted band by having an interaction induced by non-linear electrical effects occurring within components of the microwave plumbing assemblage.
Further, the interdigital filters used in such assemblage in the field trials were significant sources of the interference appearing at the input of the receiver of the duplex systems tested in these trials. Those filters were made of aluminum plated with a maximum thickness of about 80 microinches of silver on copper plating underlain by nickel underplate.